There is an affinity between refrigerants and lubricants such that one is normally present with the other in refrigeration systems. As a result, when pumping oil into the lubrication system, the pressure drop at the suction side of the oil pump can cause outgassing such that a two phase flow is being pumped. While oil is incompressible, the two phase flow is compressible. A problem associated with outgassing and two phase flow is a decreased ability to prime such that 8-12 minutes may elapse before an adequate pumping pressure is reached. Maintaining lubricant flow during transients which are dominated by two phase flow is as important as oil delivery in steady state operation. The transients can be the result of system pressure fluctuations acting on the sump as well as the result of heating, as by friction, all of which can produce outgassing and thereby two phase flow. The synthetic oils, such as polyol ester oils (POE), used with the new refrigerants release dissolved refrigerants much more rapidly than mineral oil and, as a result, the maintenance of adequate oil pressure under transient conditions is more difficult. A characteristic of the POE oils is that because they are more polar they do not "wet" the surfaces of the more polar metals such as aluminum or tin as well as mineral oil. As a result, more polar metals must be supplied continuously with a flow of oil from the pump i.e. with POE oils the pump must replenish the oil film with minimal interruption.
Commonly assigned U.S. patent application Ser. No. 85,793 filed Jul. 6, 1993, and now U.S. Pat. No. 5,295,815, addresses a portion of the problem. Specifically, that invention teaches the drawing in of unagitated oil. The oil pickup is isolated from the chamber where rotation of the members produces outgassing and centrifugal separation which tends to force oil from the oil inlet. However, that invention does not address pumping a two phase flow in the sense of specifically accommodating the two phase flow as the pumped fluid.